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(11) | EP 0 404 461 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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Gradient coil assemblies for generating magnetic field gradients across a region Gradientspulenaufbauten für die Erzeugung von Magnetfeldgradienten über einem Bereich Assemblage de bobines à gradient pour générer les gradients d'un champ magnétique à travers un domaine |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] This invention relates to gradient coil assemblies for generating magnetic field gradients across a region. It finds particular application in conjunction with establishing gradient magnetic fields in magnetic resonance imaging techniques and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in spectroscopy and other processes and apparatus in which accurately predictable magnetic field gradients are to be established or maintained.
[0002] In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Gradient fields are conventionally applied as a series of gradient pulses with preselected profiles. These pulses excite magnetic resonance, phase and frequency encode the resonance, and cause phase and frequency encoded magnetic resonance signals to be emitted.
[0003] More specifically, the gradient magnetic pulses are applied to select and encode the magnetic resonance signals. In some embodiments, the magnetic field gradients are applied to select one or more planes or slices to be imaged. Gradient field pulses are also applied for selectively modifying the uniform magnetic field to encode frequency and phase into the magnetization, hence the resonance signals in order to identify a spatial location.
[0004] The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region. The accuracy of the resultant image representation, among other factors, is dependent upon the accuracy with which the actually applied magnetic field gradient pulses conform to selected gradient pulse profiles.
[0005] Heretofore, linear magnetic field gradients have been produced by cylindrical gradient field coils. Discrete coils were wound in a bunched or distributed fashion on a large diameter hollow right cylinder tube, commonly 65 centimeters in diameter or larger. Conventional bunched geometries include Maxwell or modified-Maxwell pair for z-gradient production and single or multi-arc Golay saddle coils for x and y gradient production. The coils are normally wound in a series arrangement and positioned to give a magnetic field profile with the desired linearity over a predefined volume. The distributive windings on the right angle cylinders are generally wound and in pairs and driven anti-symmetrically. The coils are driven in an anti-symmetric manner such that only odd field derivatives are non-zero at the coil origin. The first derivative is the field gradient while the third and higher order derivatives represent distortion. If the diameter of the cylinder and coil placement are chosen properly, the third derivative is cancelled at the origin making the fifth derivative the first distortion term.
[0006] The conventional gradient coils are constructed of copper buss bar or multi-strand wire that is wound on a glass reinforced plastic tube. The inductance, which is related to the stored magnetic energy, is critical in gradient coil design. The inductance relates to how quickly the coil can switch the gradient field on and off with a given supply of voltage. Large inductances, as are typical in wound cylindrical coils, limit the switching speed of the gradient magnetic fields.
[0007] EP-A-0217520 relates to a magnetic imaging apparatus having a magnet providing a generally rectangular opening for receiving the object to be imaged and having first and second spaced apart substantially planar pole faces which are facing each other. First and second coil assemblies are disposed in the opening and provide a main magnetic field. First and second spaced apart substantially planar gradient field structures are disposed in the opening adjacent the pole faces and are adherent to the pole faces.
[0008] For maximum efficiency, it would be advantageous to reduce the diameter of the gradient coil cylinders to be as close as possible to the subject, provided gradient linearity can be maintained. The required energy for field gradient production varies roughly as a fifth power of a gradient coil cylinder diameter in free space. In an actual magnetic resonance imager, the gradient coils interact with other adjoining structures such as radiation shields of superconducting magnets, making the relationship somewhat greater than the fifth power. Although reducing the coil size could have a dramatic effect on power consumption, reducing the cylinder diameter below 65 centimeters would be too restrictive to receive patients for full body scans. Although less power consumptive, a single planar surface gradient coil, by contrast, suffers from poor gradient uniformity or field linearity. Images using a single planar gradient coil require geometric distortion correction for head size volumes and larger.
[0009] It is an object of the present invention to provide a gradient coil assembly for generating magnetic field gradients across a region wherein the above problems are overcome.
[0010] According to the invention there is is provided a gradient coil assembly for generating magnetic field gradients across a region comprising: coil means; and current supply means for supplying current to the coil means, characterised in that: said coil means comprises a first coil loop array disposed along a surface, a second coil loop array disposed along the surface, a third coil loop array disposed between the first and second coil loop arrays along the surface, and a fourth coil loop array disposed along the surface; and said current supply means supplies current to the coil loop arrays such that the first and fourth coil loop arrays have a net clockwise current flow and the second and third coil loop arrays have a net counterclockwise current flow.
[0011] One advantage of a gradient coil assembly embodying the present invention is that it achieves high magnetic field gradient strengths with relatively short rise times.
[0012] Another advantage of a gradient coil assembly embodying the present invention is that it provides improved linearity in the magnetic field gradients.
[0013] Yet another advantage is that it minimizes stored energy, hence, minimizes gradient rise time for a given geometry and desired magnetic field pattern.
[0015] One gradient coil assembly in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings in which:-
Figure 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus incorporating the gradient coil assembly;
Figure 2 illustrates a first part of the assembly for generating a magnetic field gradient in a first direction; and
Figure 3 illustrates a second part of the assembly for generating a magnetic field gradient in a second direction perpendicular to the first.
[0016] Referring to Figure 1, the magnetic resonance imaging apparatus comprises a main magnetic field means 10 for generating a substantially uniform magnetic field longitudinally through an examination or image region 12. The main magnetic field means includes a plurality of main field magnets 14 which are operated under control of a magnetic field control means and power supply 16. Preferably, the main magnetic field is a strong uniform field that is aligned with a z or longitudinal axis.
[0017] A gradient field means 20 selectively creates gradient magnetic fields across the examination region 12. The gradient field means includes a y gradient coil assembly 22 which applies gradients along a y axis, preferably a vertical axis transverse to the z axis. The y gradient coil assembly includes an upper coil winding assembly 22a and a lower coil winding assembly 22b. Analogously, an x gradient coil assembly 24 selectively applies gradients in the x direction - horizontal, transverse to the z axis in the illustrated embodiment. The x gradient coil assembly is again mounted on two parallel surfaces 24a, 24b on opposite sides of the examination region. A z gradient coil assembly 26 includes a pair of annular or other conventional coils 26a, 26b for creating gradients along the z axis.
[0018] More specifically to the preferred embodiment, each of the x and y gradient coil assembly includes a plurality of symmetrically arranged windings or coil loops described in greater detail in conjunction with FIGURE 2 below. Each of these windings are disposed on one of four smooth surfaces, preferably planes. The surfaces of the y gradient coils 22a, 22b are larger in transverse direction than the surfaces of the x gradient coils 24a, 24b. The difference in width is selected in accordance with the cross sectional aspect ratio of human patients to optimize the shape and size of the examination region.
[0019] A current control means 28 selectively supplies electrical currents to each loop of the x, y, and z gradient coils. The current is supplied to the coil loops such that the currents flow through each loop in either a clockwise or counterclockwise direction. More specific to the preferred embodiment, half of the coil arrays on each planar surface have a clockwise current flow and the other half will have a counterclockwise current flow. A gradient field control means 30 controls the current control means 28 to cause it to apply appropriate current pulses to the gradient field windings to cause selected gradient pulses.
[0020] A resonance excitation means 40 includes a radio frequency transmitter 42 for generating radio frequency pulses of the appropriate frequency spectrum for inducing resonance in selected dipoles disposed in the examination region. The radio frequency transmitter is connected to a radio frequency antenna 44 disposed adjacent the examination region for transmitting radio frequency pulses into a region of interest of the patient or other subject in the examination region. Although the radio frequency antenna is illustrated as being disposed peripherally around the gradient coil assemblies, it is to be appreciated that such antenna may also be disposed within the gradient coil assemblies. For example, a surface coil may be positioned contiguous to an examined patient or subject for controllably inducing magnetic resonance in a selected contiguous region of the patient.
[0021] A magnetic resonance receiving means 50 includes the radio frequency coil 44 which receives, as well as transmits, radio frequency signals in the illustrated embodiment. For other studies, separate transmit and receive coils are used. For example, receive only surface coils may be disposed contiguous to a selected region of the patient to receive resonance induced by the radio frequency coil 44. A radio frequency receiver 52 receives the radio frequency signals from the antenna. The received radio frequency signals are demodulated and reconstructed into an image representation. More specifically, a Fourier transform means 54 transforms magnetic resonance signals into an image representation for storage in an image memory means 56. As is conventional in the art, the image may represent a planar slice through the patient, an array of parallel planar slices, a three dimensional volume, or the like. A display means 58, such as a video monitor, provides a man-readable display of the resultant image. Other conventional processing equipment, which is conventional in the art, is omitted from the illustration for simplicity.
[0022] Each of the x and y gradient coil assemblies includes two surfaces that are symmetric with respect to a central plane. The illustrated planar surfaces are parallel to the z axis plane of symmetry. Each surface includes a sheet of non-magnetic and non-conducting material on which a distributed conductive coil assembly is mounted. The distributed coil patterns represent a discrete approximation of an optimal, continuous current distribution. The optimal current is computed to yield minimum stored energy for the desired magnetic field pattern. For the preferred imaging embodiment, the magnetic field pattern is a linear gradient that is symmetric about a geometric center line along the z axis, midway between the x gradient coil surfaces and the y gradient coil surfaces. That is, the field is symmetric about (x,y), (x,z), and (y,z) planes of symmetry. This magnetic field pattern for the biplanar y gradient coil is defined by the equation:
where
In the equations, "a" represents the half separation distance between the coils, λk are the solved for Lagrange multipliers, Bzc (Xj, Yj, Zj) are the desired values of the magnetic flux densities at points (Xj, Yj, Zj), α and β are Fourier transform variables, jxa are the jza are x and z components of the continuous surface current density function, and Wm is the stored magnetic energy. Analogous equations represent the solution for a biplanar y gradient coil or z gradient coil.
[0023] In the preferred magnetic resonance imaging y gradient coil, the values of the magnetic flux density change linearly in the y direction and are constant in the x and z direction. For the x gradient coil, the magnetic flux changes linearly with distance in the x direction and is constant in the y and z directions. The surface current densities are mirror images on the oppositely disposed coil surfaces such that the gradients vary from positive at one side through zero at the geometric center to a negative value at the opposite end. Alternately, instead of having the zero gradient point in the geometric center of the coils, it may be offset. For example, to facilitate imaging of the spinal region, the magnetic flux may be designed to have the zero gradient displaced from the lower y gradient coil surface a short distance such that it falls within the center of the spine.
[0024] With particular reference to FIGURE 2, the current density is approximated by a first loop array 60 and a second coil loop array 62 disposed symmetrically about a line of symmetry 64. In the preferred embodiment, the line of symmetry is the intersection between the illustrated plane and a plane of symmetry about which all coil assemblies are symmetric. A first order correction for irregularities in the gradient generated by the first and second loop arrays is made by a third coil loop array 66 and a fourth coil loop array 68. The third and fourth coil loop arrays are also disposed symmetrically about the line of symmetry but are smaller than the first and second coil loop arrays. Each of the four coil loop assemblies include a plurality of generally circular loops which are disposed symmetrically relative to the z axis. Theoretically, the first and second coil loop arrays would include additional peripheral current flow paths carrying progressively smaller amounts of current extending out to infinity. However, physical size constraints put a limit on the maximum size of these coil arrays.
[0025] To provide a higher order or more accurate approximation of a linear magnetic field gradient, additional small coil loop arrays may be provided. Such additional coil loop arrays are disposed in pairs symmetric about the z axis between the first and second coil arrays and the line of symmetry. Such additional coil arrays are normally smaller in diameter than the third and fourth coil arrays. The actual number and placement of these additional coil loop arrays is determined by the magnetic field pattern to be achieved and the accuracy with which it is to be approximated.
[0026] The opposite y planar surface 22b has, in the embodiment in which the zero gradient is centered, an identical winding pattern. The current control means 28 applies electrical currents to the coils such that the current flow about the first and second coil loop arrays is opposite, e.g. clockwise and counterclockwise, respectively. The current control means further applies currents to the third and fourth coil loop arrays such that the current flows in opposite directions relative to each other and relative to the closest of the first and second coil loop arrays, e.g. counterclockwise and clockwise. For example, current may flow clockwise through the first and fourth coil loop array and counterclockwise through the second and third coil loop array on the upper plane 22a. On the lower y coil surface 22b, current flow is the opposite. That is, current flows counterclockwise through the first and fourth coil loop arrays and clockwise through the second and third current loop arrays.
[0027] FIGURE 3 illustrates a preferred coil winding pattern for the x gradient coil. The coil is substantially the same as the y gradient coil but compressed in width to accommodate the shorter dimension.
[0028] Optionally, additional planar coil winding patterns may be provided adjacent the outside of the gradient coils away from the examination region. These additional windings may provide a shielding that cancels the flux density exterior to the biplanar coils. As discussed above, the coils may also be asymmetric in such a manner that the center of the linear field gradient is offset from the geometric center of the examination region. As yet another option, the annular z gradient coils may be replaced with planar coils, where access constraints permit.
1. A gradient coil assembly (20) for generating magnetic field gradients across a region
(12) comprising: coil means (22a, 22b, 24a, 24b, 26a, 26b); and current supply means
(28, 30) for supplying current to the coil means, characterised in that: said coil
means comprises a first coil loop array (62) disposed along a surface (22a), a second
coil loop array (60) disposed along the surface (22a), a third coil loop array (68)
disposed between the first (62) and second (60) coil loop arrays along the surface
(22a), and a fourth coil loop array (66) disposed along the surface (22a); and said
current supply means (28,30) supplies current to the coil loop arrays (62, 60, 68,
66) such that the first (62) and fourth (66) coil loop arrays have a net clockwise
current flow and the second (60) and third (68) coil loop arrays have a net counterclockwise
current flow.
2. An assembly according to Claim 1 wherein: said surface (22a) is a smooth surface;
said third coil loop array (68) is smaller than the first (62) and second (60) coil
loop arrays; said fourth coil loop array (66) is smaller than the first (62) and second
(60) coil loop arrays; and said first, second, third and fourth coil loop arrays (62,
60, 68, 66) are disposed substantially in linear alignment with each other.
3. An assembly according to Claim 2 further including: a fifth coil loop array (62) disposed
along a second smooth surface (22b); a sixth coil loop array (60) disposed along the
second smooth surface (22b); a seventh coil loop array (68) smaller than the fifth
(62) and sixth (60) coil loop arrays disposed between the fifth (62) and sixth (60)
coil loop arrays along the second smooth surface (22b); and an eighth coil loop array
(66) smaller than the fifth (62) and sixth (60) coil loop arrays disposed along the
second smooth surface (22b), the fifth, sixth, seventh, and eighth coil loop arrays
(62, 60, 68, 66) being disposed substantially in linear alignment parallel to the
first, second, third, and fourth coil loop arrays (62, 60, 68, 66).
4. An assembly according to Claim 3 wherein the current supply means (28, 30) supplies
current to the fifth, sixth, seventh, and eighth coil loop arrays (62, 60, 68, 66)
such that the fifth (62) and eighth (66) coil loop arrays have a net counterclockwise
current flow and the sixth (60) and seventh (68) coil loop arrays have a net clockwise
current flow.
5. An assembly according to Claim 3 or Claim 4 wherein the first (62), third (68), fifth
(62), and seventh (68) coil loop arrays are disposed on one side of a plane (xy) of
symmetry that extends through the first mentioned and second surfaces (22a, 22b) and
the second (60), fourth (66), sixth (60) and eighth (66) coil loop arrays are disposed
on the opposite side of said plane of symmetry (xy) symmetrically to the first (62),
third (68), fifth (62), and seventh (68) coil loop arrays.
6. An assembly according to any one of Claims 3 to 5 wherein the first mentioned and
second smooth surfaces (22a, 22b) are planar and parallel.
7. An assembly according to Claim 6 when dependent on Claim 5 wherein said plane of symmetry
(xy) extends substantially perpendicular to said first mentioned and second surfaces
(22a, 22b).
8. An assembly according to Claim 7 further including a first plurality of further coil
loop arrays (62, 60, 68, 66) disposed along a third planar surface (24a) and a second
plurality of further coil loop arrays (62, 60, 68, 66) disposed along a fourth planar
surface (24b), the third and fourth planar surfaces (24a, 24b) being disposed generally
parallel to each other and substantially perpendicular to the first mentioned and
second surfaces (22a, 22b).
9. An assembly according to Claim 8 wherein the first and second pluralities of further
coil loop arrays (62, 60, 68, 66) are disposed on the third and fourth planar surfaces
(24a, 24b) symmetrically relative to said plane of symmetry (xy).
10. A magnetic resonance imaging apparatus comprising: main magnetic field means (10)
for generating a main magnetic field through an examination region (12); magnetic
field gradient means (20, 28, 30) for generating magnetic field gradients across the
examination region (12), the magnetic field gradient means including a gradient coil
assembly (20) according to Claim 1, said first (62) and second (60) coil loop arrays
being disposed symmetrically to each other about a line of symmetry (64), said third
(68) and fourth (66) coil loop arrays being disposed symmetrically about said line
of symmetry (64), and said fourth coil loop array (66) being disposed between the
first and second coil loop arrays (62, 60); magnetic resonance excitation means (40,
42, 44) for selectively exciting magnetic resonance of dipoles disposed in the examination
region (12); magnetic resonance signal receiving means (44, 50, 52) for receiving
magnetic resonance signals from the resonating dipoles; and reconstruction means (54,
56, 58) for reconstructing an image representation from the magnetic resonance signals.
11. An apparatus according to Claim 10 wherein said gradient coil assembly (20) further
includes: fifth (62) and sixth (60) coil loop arrays disposed symmetrically to each
other about a second line of symmetry (64) and along a second surface (22b), the second
surface (22b) being disposed substantially parallel to the first mentioned surface
(22a); and seventh (68) and eighth (66) coil loop arrays disposed symmetrically about
said second line of symmetry (64), between the fifth (62) and sixth (60) coil loop
arrays and along the second surface (22b), and said current supply means (28, 30)
supplies electric current to the fifth, sixth, seventh and eighth coil loop arrays
(62, 60, 68, 66) such that the fifth (62) and eighth (66) coil loop arrays have a
net counterclockwise current flow and the sixth (60) and seventh (68) coil loop arrays
have a net clockwise current flow.
12. An apparatus according to Claim 11 further including third (24a) and fourth (24b)
surfaces disposed parallel to each other and generally orthogonal to the first mentioned
and second surfaces (22a, 22b), the third and fourth surfaces (24a, 24b) each having
a plurality of coil loop arrays (62, 60, 68, 66) mounted thereon for selectively generating
a gradient magnetic field therebetween.
13. An apparatus according to Claim 10 or Claim 11 or Claim 12 wherein the main magnetic
field extends longitudinally along a z axis and the or each line of symmetry (64)
extends parallel to an x axis such that a magnetic field gradient along a y axis is
created.
14. An apparatus according to Claim 13 wherein the first, second, third and fourth coil
loop arrays (62, 60, 68, 66) each intersect a plane of symmetry (zy) which is parallel
to the y and z axes and are each symmetric thereabout.
1. Gradientenspulenanordnung (20) zum Erzeugen von Magnetfeldgradienten durch eine Region
(12), aufweisend: eine Spuleneinrichtung (22a, 22b, 24a, 24b, 26a, 26b); und eine
Stromzufuhreinrichtung (28, 30) zur Stromzuführung zur Spuleneinrichtung, dadurch
gekennzeichnet, daß: die Spuleneinrichtung eine erste Spulenschleifenanordnung (62)
aufweist, die entlang einer Oberfläche (22a) angeordnet ist, eine zweite Spulenschleifenanordnung
(60), die entlang der Oberfläche (22a) angeordnet ist, eine dritte Spulenschleifenanordnung
(68), die entlang der Oberfläche (22a) zwischen der ersten (62) und zweiten 60) Spulenschleifenanordnung
angeordnet ist, und eine vierte Spulenschleifenanordnung (66), die entlang der Oberfläche
(22a) angeordnet ist; und daß die Stromzufuhreinrichtung (28, 30) den Spulenschleifenanordnungen
(62, 60, 68, 66) derart Strom zuführt, daß die erste (62) und vierte (66) Spulenschleifenanordnung
einen Gesamtstromfluß im Uhrzeigersinn aufweisen und die zweite (60) und dritte (68)
Spulenschleifenanordnung einen Gesamtstromfluß entgegen dem Uhrzeigersinn aufweisen.
2. Anordnung nach Anspruch 1, in welcher: die Oberfläche (22a) eine gleichmäßige Fläche
ist; die dritte Spulenschleifenanordnung (68) kleiner als die erste (62) und zweite
(60) Spulenschleifenanordnung sind; die vierte Spulenschleifenanordnung (66) kleiner
als die erste (62) und zweite (60) Spulenschleifenanordnung ist; und die erste, zweite,
dritte und vierte Spulenschleifenanordnung (62, 60, 68, 66) im wesentlichen in linearer
Ausrichtung zueinander angeordnet sind.
3. Anordnung nach Anspruch 2, ferner aufweisend: eine fünfte Spulenschleifenanordnung
(62), die entlang einer zweiten gleichmäßigen Oberfläche (22b) angeordnet ist; eine
sechste Spulenschleifenanordnung (60), die entlang der zweiten gleichmäßigen Oberfläche
(22b) angeordnet ist; eine siebte Spulenschleifenanordnung (68), die kleiner als die
fünfte (62) und sechste (60) Spulenschleifenanordnung ist und zwischen der fünften
(62) und sechsten (60) Spulenschleifenanordnung entlang der zweiten gleichmäßigen
Oberfläche (22b) angeordnet ist; und eine achte Spulenschleifenanordnung (66), die
kleiner als die fünfte (62) und sechste (60) Spulenschleifenanordnung ist und entlang
der zweiten gleichmäßigen Oberfläche (22b) angeordnet ist, wobei die fünfte, sechste,
siebte und achte Spulenschleifenanordnung (62, 60, 68, 66) im wesentlichen in linearer
Ausrichtung parallel zur ersten, zweiten, dritten und vierten Spulenschleifenanordnung
(62, 60, 68, 66) angeordnet sind.
4. Anordnung nach Anspruch 3, in welcher die Stromzufuhreinrichtung (28, 30) der fünften,
sechsten, siebten und achten Spulenschleifenanordnung (62, 60, 68, 66) derart elektrischen
Strom zuführt, daß die fünfte (62) und achte (66) Spulenschleifenanordnung einen Gesamtstromfluß
entgegen dem Uhrzeigersinn aufweisen und die sechste (60) und siebte (68) Spulenschleifenanordnung
einen Gesamtstromfluß im Uhrzeigersinn aufweisen.
5. Anordnung nach Anspruch 3 oder 4, in welcher die erste (62), dritte (68), fünfte (62)
und siebte (68) Spulenschleifenanordnung auf einer Seite einer Symmetrieebene (xy)
angeordnet sind, die sich durch die zuerst erwähnte und zweite Oberfläche (22a, 22b)
erstreckt, und die zweite (60), vierte (66), sechste (60) und achte (66) Spulenschleifenanordnung
auf der entgegengesetzten Seite der Symmetrieebene (xy) symmetrisch zur ersten (62),
dritten (68), fünften (62) und siebten (68) Spulenschleifenanordnung angeordnet sind.
6. Anordnung nach einem der Ansprüche 3 bis 5, in welcher die zuerst erwähnte und zweite
gleichmäßige Oberfläche (22a, 22b) planar und parallel sind.
7. Anordnung nach Anspruch 6 bei Rückbeziehung auf Anspruch 5, in welcher die Symmetrieebene
(xy) sich im wesentlichen senkrecht zur zuerst erwähnten und zweiten Oberfläche (22a,
22b) erstreckt.
8. Anordnung nach Anspruch 7, ferner aufweisend eine erste Mehrzahl von weiteren Spulenschleifenanordnungen
(62, 60, 68, 66), die entlang einer dritten planaren Oberfläche (24a) angeordnet sind,
und eine zweite Mehrzahl von weiteren Spulenschleifenanordnungen (62, 60, 68, 66),
die entlang einer vierten planaren Oberfläche (24b) angeordnet sind, wobei die dritte
und vierte planare Oberfläche (24a, 24b) im wesentlichen parallel zueinander und im
wesentlichen senkrecht zu der zuerst erwähnten und zweiten Oberfläche (22a, 22b) angeordnet
sind.
9. Anordnung nach Anspruch 8, in welcher die erste und zweite Mehrzahl von weiteren Spulenschleifenanordnungen
(62, 60, 68, 66) auf der dritten und vierten planaren Oberfläche (24a, 24b) in Bezug
auf die Symmetrieebene (xy) symmetrisch angeordnet sind.
10. Magnetresonanzabbildungsvorrichtung, aufweisend: eine Grundmagnetfeldeinrichtung (10)
zum Erzeugen eines magnetischen Grundfeldes durch eine Untersuchungsregion (12); eine
Magnetfeldgradienteneinrichtung (20,28,30) zum Erzeugen von Magnetfeldgradienten durch
die Untersuchungsregion (12), wobei die Magnetfeldeinrichtung eine Gradientenspulenanordnung
(20) nach Anspruch 1 umfaßt, wobei die erste (62) und zweite (60) Spulenschleifenanordnung
symmetrisch zueinander um eine Symmetrielinie (64) angeordnet sind, die dritte (68)
und vierte (66) Spulenschleifenanordnung symmetrisch um die Symmetrielinie (64) angeordnet
sind, und die vierte Spulenschleifenanordnung (66) zwischen der ersten und zweiten
Spulenschleifenanordnung (62 ,60) angeordnet ist; eine Magnetresonanzanregungseinrichtung
(40, 42, 44) zur selektiven Anregung der Magnetresonanz von in der Untersuchungsregion
(12) angeordneten Dipolen; eine Magnetresonanzsignal-Empfangseinrichtung (44, 50,
52) zum Empfang von Magnetresonanzsignalen von den resonierenden Dipolen; und eine
Rekonstruktionseinrichtung (54, 56, 58) zur Rekonstruktion einer Bilddarstellung aus
den Magnetresonanzsignalen.
11. Vorrichtung nach Anspruch 10, in welcher die Gradientenspulenanordnung (20) ferner
aufweist: fünfte (62) und sechste (60) Spulenschleifenanordnungen, die symmetrisch
zueinander um eine zweite Symmetrielinie (64) und entlang einer zweiten Oberfläche
(22b) angeordnet sind, wobei die zweite Oberfläche (22b) im wesentlichen parallel
zur zuerst erwähnten Oberfläche (22a) angeordnet ist, und eine siebte (68) und achte
(66) Spulenschleifenanordnung, die symmetrisch um die zweite Symmetrielinie (64) zwischen
der fünften (62) und sechsten (60) Spulenschleifenanordnung und entlang der zweiten
Oberfläche (22b) angeordnet sind, und in welcher die Stromzufuhreinrichtung (28, 30)
der fünften, sechsten, siebten und achten Spulenschleifenanordnung (62, 60, 68, 66)
derart elektrischen Strom zuführt, daß die fünfte (62) und achte (66) Spulenschleifenanordnung
einen Gesamtstromfluß entgegen dem Uhrzeigersinn aufweisen und die sechste (60) und
siebte (68) Spulenschleifenanordnung einen Gesamtstromfluß im Uhrzeigersinn aufweisen.
12. Vorrichtung nach Anspruch 11, ferner aufweisend eine dritte (24a) und vierte (24b)
Oberfläche, die parallel zueinander und im wesentlichen senkrecht zur erstgenannten
und zweiten Oberfläche (22a, 22b) angeordnet sind, wobei die dritte und vierte Oberfläche
(24a, 24b) jeweils mehrere Spulenschleifenanordnungen (62, 60, 68, 66) aufweisen,
die auf ihnen zur selektiven Erzeugung eines magnetischen Gradienten zwischen sich
angebracht sind.
13. Vorrichtung nach Anspruch 10 oder 11 oder 12, in welcher das magnetische Grundfeld
sich longitudinal entlang einer z-Achse erstreckt und sich die oder jede Symmetrielinie
(64) parallel zu einer x-Achse erstreckt, derart, daß ein Magnetfeldgradient entlang
einer y-Achse erzeugt wird.
14. Vorrichtung nach Anspruch 13, in welcher die erste, zweite, dritte und vierte Spulenschleifenanordnung
(62, 60, 68, 66) jeweils ein Symmetrieebene (xy) schneiden, die parallel zur y- und
x-Achse liegt, und jeweils symmetrisch um diese sind.
1. Ensemble (20) de bobines de gradient destiné à créer des gradients de champ magnétique
dans une région (12), comprenant un dispositif à bobines (22a, 22b, 24a, 24b, 26a,
26b), et un dispositif (28, 30) d'alimentation en courant destiné à transmettre un
courant au dispositif à bobines, caractérisé en ce que le dispositif à bobines comprenant
une première matrice (62) de boucles de bobine disposée le long d'une surface (22a),
une seconde matrice (60) de boucles de bobine disposée le long de la surface (22a,
une troisième matrice (68) de boucles de bobine disposée entre la première (62) et
la seconde (60) matrice de boucles de bobine le long de la surface (22a), et une quatrième
matrice (66) de boucles de bobine placée le long de la surface (22a), et le dispositif
d'alimentation en courant (28, 30) transmet un courant aux matrices (62, 60, 68, 66)
de boucles de bobine afin que la première (62) et la quatrième (66) matrice de boucles
de bobine présentent une circulation résultante du courant dans le sens des aiguilles
d'une montre et que la seconde (60) et la troisième (68) matrice de boucles de bobine
présentent une circulation résultante du courant dans le sens contraire des aiguilles
d'une montre.
2. Ensemble selon la revendication 1, dans lequel ladite surface (22a) est une surface
lisse, la troisième matrice (68) de boucles de bobine est plus petite que la première
(62) et la seconde (60) matrice de boucles de bobine, la quatrième matrice (66) de
boucles de bobine est plus petite que la première (62) et la seconde (60) matrice
de boucles de bobine, et les première, seconde, troisième et quatrième matrices de
boucles de bobine (62, 60, 68, 66) sont disposées pratiquement sous forme alignée
linéairement les unes par rapport aux autres.
3. Ensemble selon la revendication 2, comprenant en outre une cinquième matrice (62)
de boucles de bobine placée le long d'une seconde surface lisse (22b), une sixième
matrice (60) de boucles de bobine placée le long de la seconde surface lisse (22b),
une septième matrice (68) de boucles de bobine plus petite que la cinquième (62) et
la sixième (60) matrice de boucles de bobine, disposée entre les cinquième (62) et
sixième (60) matrice de boucles de bobine le long de la seconde surface lisse (22b),
et une huitième matrice (66) de boucles de bobine inférieure à la cinquième (62) et
à la sixième (60) matrice de boucles de bobine et disposée le long de la seconde surface
lisse (22b), les cinquième, sixième, septième et huitième matrices (62, 60, 68, 66)
de boucles de bobine étant disposées pratiquement dans l'alignement linéaire parallèlement
aux première, seconde, troisième et quatrième matrices (62, 60, 68, 66) de boucles
de bobine.
4. Ensemble selon la revendication 3, dans lequel le dispositif d'alimentation en courant
(28, 30) transmet un courant aux cinquième, sixième, septième et huitième matrices
(62, 60, 68, 66) de boucles de bobine afin que la cinquième (62) et la huitième (66)
matrice de boucles de bobine présentent une circulation résultante de courant dans
le sens contraire des aiguilles d'une montre et que la sixième (60) et la septième
(68) de matrice de boucles de bobine présentent une circulation résultante du courant
dans le sens des aiguilles d'une montre.
5. Ensemble selon la revendication 3 ou 4, dans lequel les première (62), troisième (68),
cinquième (62) et septième (68) matrices de boucles de bobine sont placées d'un premier
côté d'un plan (xy) de symétrie passant par la première et la seconde surface précitées
(22a, 22b), et les seconde (60), quatrième (66), sixième (60) et huitième (66) matrices
de boucles de bobine sont placées du côté opposé du plan de symétrie (xy) symétriquement
aux première (62), troisième (68), cinquième (62) et septième (68) matrices de boucles
de bobine.
6. Ensemble selon l'une quelconque des revendications 3 à 5, dans lequel la première
et la seconde surface lisse (22a, 22b) sont planes et parallèles.
7. Ensemble selon la revendication 6 lorsqu'elle dépend de la revendication 5, dans lequel
le plan de symétrie (xy) passe en direction pratiquement perpendiculaire à la première
et à la seconde surface (22a, 22b).
8. Ensemble selon la revendication 7, comprenant en outre un premier ensemble de matrices
supplémentaires (62, 60, 68, 66) de boucles de bobine disposé le long d'une troisième
surface plane (24a), et un second ensemble de matrices supplémentaires (62, 60, 68,
66) de boucles de bobine disposé le long d'une quatrième surface plane (24b), la troisième
et la quatrième surface plane (24a, 24b) étant disposées en direction sensiblement
parallèle l'une à l'autre et sensiblement perpendiculaire à la première et à la seconde
surface (22a, 22b).
9. Ensemble selon la revendication 8, dans lequel le premier et le second ensemble de
matrices supplémentaires (62, 60, 68, 66) de boucles de bobine sont disposés sur la
troisième et la quatrième surface plane (24a, 24b) symétriquement par rapport au plan
de symétrie (xy).
10. Appareil de formation d'image de résonance magnétique, comprenant un dispositif (10)
générateur d'un champ magnétique principal dans une région d'examen (12), un dispositif
(20, 28, 30) à gradients de champ magnétique destiné à créer des gradients de champ
magnétique dans la région d'examen (12), le dispositif à gradients de champ magnétique
comprenant un ensemble (20) à bobines de gradient selon la revendication 1, la première
(62) et la seconde (60) matrice de boucles de bobine étant disposées symétriquement
l'une de l'autre par rapport à un axe de symétrie (64), la troisième (68) et la quatrième
(66) matrice de boucles de bobine étant disposées symétriquement par rapport à l'axe
de symétrie (64), et la quatrième matrice (66) de boucles de bobine étant placée entre
la première et la seconde matrice (62, 60) de boucles de bobine, un dispositif (40,
42, 44) d'excitation sélective de la résonance magnétique de dipôles disposés dans
la région d'examen (12), un dispositif (44, 50, 52) de réception de signaux de résonance
magnétique destiné à recevoir des signaux de résonance magnétique provenant des dipôles
qui résonnent, et un dispositif (54, 56, 58) de reconstruction d'une représentation
d'image à partir des signaux de résonance magnétique.
11. Appareil selon la revendication 10, dans lequel l'ensemble (20) à bobines de gradient
comporte en outre une cinquième (62) et une sixième (60) matrice de boucles de bobine
disposées symétriquement l'une par rapport à l'autre autour d'un second axe de symétrie
(64), et le long d'une seconde surface (22b), la seconde surface (22b) étant disposée
en direction sensiblement parallèle à la première surface (22a), et une septième (68)
et une huitième (66) matrice de boucles de bobine disposées symétriquement par rapport
au second axe de symétrie (64), entre la cinquième (62) et la sixième (60) matrice
de boucles magnétiques et le long de la seconde surface (22b), et le dispositif d'alimentation
en courant (28, 30) transmet un courant électrique aux cinquième, sixième, septième
et huitième matrices (62, 60, 68, 66) de boucles magnétiques afin que la cinquième
(62) et la huitième (66) matrice de boucles magnétiques présentent une circulation
résultante du courant dans le sens contraire des aiguilles d'une montre et que la
sixième (60) et la septième (68) matrice de boucles magnétiques présentent une circulation
résultante du courant dans le sens des aiguilles d'une montre.
12. Appareil selon la revendication 11, comprenant en outre une troisième (24a) et une
quatrième (24b) surface qui sont disposées parallèlement l'une à l'autre et qui sont
perpendiculaires de façon générale aux première et seconde surfaces précitées (22a,
22b), la troisième et la quatrième surface (24a, 24b) ayant chacune plusieurs matrices
(62, 60, 68, 66) de boucles magnétiques montées sur la surface et destinées à créer
sélectivement un champ magnétique de gradient entre elles.
13. Appareil selon la revendication 10, 11 ou 12, dans lequel le champ magnétique principal
est disposé longitudinalement suivant un axe z et l'axe ou chaque axe de symétrie
(64) est parallèle à un axe x afin qu'un gradient de champ magnétique soit créé le
long d'un axe y.
14. Appareil selon la revendication 13, dans lequel les première, seconde, troisième et
quatrième matrices (62, 60, 68, 66) de boucles de bobine recoupent chacune un plan
de symétrie (zy) qui est parallèle aux axes y et z et sont chacune symétriques par
rapport à lui.